1. Field of the Invention
This invention relates to telephone system equipment that is used at the subscriber's end of a subscriber's line on conjunction with a telephone station, and in particular to a station identification circuit arrangement for ANI (Automatic Number Identification) mark circuits which are detected by central office equipment when a call is initiated by one of four parties on a four-party line.
2. Discussion of the Relevant Art
ANI mark circuits (also referred to as station identification circuits) are used with plural party lines to identify toll calls dialed by a selected party (customarily the tip party as distinguished from the ring party) for billing purposes in an automatic number identification system.
Prior to this invention, it has generally been the practice to wire ringer isolator and ANI mark circuits directly into the telephone sets. It also has been a custom to incorporate the ANI mark circuits into the telephone sets at the time of manufacture, but to make the circuit connections in such a way that the ANI mark circuits remain inactive or disabled until such time that it is desired to install the telephone at a tip party station on a two-party line.
Because of this custom, service calls are frequently needed when a telephone set is later assigned to a new or different subscriber for the purpose of converting the set in some cases to single party or ring party use and in other cases to tip party use. Many times existing telephones already installed are not equipped with ANI mark circuits. If conversion is required for tip party use, a service call is required, and physical access to the telephone is needed in order to make a modification to utilize a circuit arrangement which must be installed in the instrument itself. In addition, house calls will be required to equip subscriber's telephones with the type of ringer isolator that is installed in the telephone itself.
In order to avoid the requirement of obtaining access to the physical telephone instrument, many circuits have been devised to provide the ANI mark equipment in such a way that it need not be physically housed in the instrument itself. One example of such an ANI mark circuit is shown in FIG. 2 of U.S. Pat. No. 4,001,512 issued to D. F. Proctor on Jan. 4, 1977. Another example of this type of circuit is disclosed in U.S. Pat. No. 4,054,942 issued to C. W. Chambers, Jr., on Oct. 18, 1977.
Both of these prior ANI mark circuits may be installed without requiring access to the subscriber's telephone. As a result no changes are required in the telephone itself or in the wiring to the telephone to divert the telephone station to tip party use on a two-party line. The ANI mark circuits described in the above-mentioned patents, however, have serious drawbacks.
In both patents, for example, the circuit designs are such that insertion or line losses are created by electrical components which are connected in the subscriber loop itself in series with one or both of the different ring conductors. In Pat. No. 4,054,942 and in one of the embodiments of Pat. No. 4,001,512 there is the additional problem of electrical noise which is created by grounding one of the two subscriber line conductors (usually the tip conductor) through a resistor, but not the other conductor in the course of establishing the ground mark.
In another type of prior ANI mark circuit, the base-emitter junction of a bipolar transistor is connected across a resistor that is in series with the tip conductor of the tip party's drop, and the collector of the transistor is connected to ground through a resistor (usually 2.6 K ohms). With this circuit design, loop current flow in the tip party's loop circuit turns on the transistor to create a difference in the current flowing in the tip and ring conductors. Resulting current difference is sensed by ANI equipment in the central office. This ANI station identification circuit is referred to as the differential type and cannot be employed with the longitudinal type of identification in which a current path to ground is sensed at the central office to identify the calling party as the tip party.
Examples of prior ringer isolator circuits are described in U.S. Pat. No. 3,916,111 issued to H. W. Ott on Oct. 28, 1975, U.S. Pat. No. 3, 303,289 issued to M. S. Hawley, et al on Feb. 7, 1967 and U.S. Pat. No. 3,230,316 issued to P. M. Hunt on Jan. 18, 1966.
These patented ringer isolator circuits for the most part are normally wired into the telephone set itself and therefore present a problem of obtaining access to the telephone to make the installation as mentioned earlier. All of the prior devices are not suitable for use on a four-party line where each party must be individually identified.
The present invention avoids the foregoing problems and drawbacks as well as offering additional advantages as will become apparent from the following summary and description.